1) when engine runs - never stop engine via turning the key! You should use the fuel tap. Close them while engine is runnig. After a short period of time (1-2min) the engine is off.
Now you do not have any fuel in the caburateur, which could loose flammability.
2) caburateur configuration
Engine must be hot! Let it run 10min!
Stand on the left side of your GILERA. Better take of the side covering if you own a RC600C. Look on the caburateur. you will see a long metal screw at that point of the caburateur, where the gas throttle is connected.
Here you can change the idle gas (the way gas is "injected" during N-gear is in and you donot touch the gas throttle). 1cm deeper and 3-4cm left of it, just before the caburateur is connected to the flat head, ist a screw which could not be seen

You can only see the hole, in which it is countersinked. This screw is used for the regulation of the air-mixture.
You need an short screwdriver to change the position of the screw.
Next part will follow when I'm home - because I'm not sure how many turns you must do...